There are two types of cerium-based abrasives (hereinafter sometimes referred to simply as abrasive): an abrasive powder provided in a dried powder state; and an abrasive slurry provided in a slurry state, which is a mixture of an abrasive powder and a dispersion medium such as water. Of the two, the abrasive slurry is used for polishing as it is, while the abrasive powder is mixed with a dispersion medium such as water to prepare an abrasive slurry before it is used for polishing. For example, the abrasive slurry is used for polishing in such a manner as to be continuously or intermittently supplied between a polishing pad and a surface to be polished. And the used abrasive slurry is usually disposed of through after-treatment known as solid-liquid separation treatment. This solid-liquid separation treatment is, for example, treatment to sediment the solid content of the abrasive slurry by adding a flocculant thereto.
As abrasive slurries used for polishing, those in which abrasive particles (solid content) are in the dispersed state are preferable. An abrasive slurry having abrasive particles in the dispersed state makes it convenient, for example, to carry out polishing operations while continuously supplying the abrasive slurry, because polishing properties such as polishing rate is stabilized and the quality of the surface obtained by the polishing is also stabilized. Means for dispersing abrasive particles include, for example, stirring the abrasive slurry. However, even if the stirring permits abrasive particles to be dispersed, once the abrasive slurry is left standing still, they start to settle at the bottom of the abrasive slurry and their dispersed state is impaired. Using the abrasive slurry in which the dispersed state of the abrasive particles has been impaired is not preferable because its polishing properties such as polishing rate vary. For example, if a portion of such an abrasive slurry is supplied of which solid content, that is, cerium-based abrasive particles content is high, inconveniences of, for example, scratches being likely to occur may be caused.
To eliminate these inconveniences, in recent years, a method has been used in which an agent for inhibiting the sediment of abrasive particles in an abrasive slurry, known as dispersant, is added to the abrasive slurry. According to this method, the dispersed state of the abrasive particles brought by stirring or the like can be maintained for a longer period of time.
However, the conventional dispersants, such as sodium hexametaphosphate and ammonium polyacrylate, contain phosphorus or ammonia. And the use of these dispersants permits the abrasive slurry to contain these components, which results in increase in the trouble of after-treating the abrasive slurry used. In particular, when using no dispersant in polishing operations, polishing operators have only to do the above described solid-liquid separation treatment as the after-treatment, but on the other hand, when using a dispersant as above, it is necessary to further separate phosphorus or nitrogen compound (ammonia) from the liquid obtained by the solid-liquid separation treatment. The method for separating nitrogen component from used abrasive slurries includes, for example, ammonia stripping or nitration-denitrification.
In recent years, there has been an increasing need for high-precision polishing, such as finish polishing of glass substrates for hard disks or LCDs, in the field of manufacturing electronic materials. With this increasing need, there have also been increasing demands for abrasives of micro-particles. For powders, it is generally considered that as long as they can be fully dispersed, those with a smaller particle diameter are harder to settle out when prepared in a slurry and their dispersed state can be maintained for a longer period of time. In fact, however, for cerium-based abrasives, those with a smaller particle diameter have lower dispersion properties and are easy to coagulate. In other words, even if the particle diameter of cerium-based abrasive particles is decreased, their dispersion maintaining property is not improved; therefore, their dispersion maintaining property cannot be ensured satisfactorily simply by mixing them in a dispersion medium and stirring the mixture.
This invention has been made in the above described background, and its subject is to provide a cerium-based powder abrasive enabling a cerium-based abrasive slurry to be prepared and the abrasive slurry after use to be treated easily, simply by mixing it with a dispersion medium such as water, wherein the dispersed state of its abrasive particles is maintained for a longer period of time, and the method for manufacturing the same.